This invention relates generally to new ester compounds, liquid crystal compositions containing the ester compound and methods of preparing the new ester compounds, and more particularly to (2'-chloro-4'-alkylphenyl)-3-cyano-4-n-alkoxybenzoates.
Electro-optical display elements including liquid crystal compositions have been put into a variety of practical uses, in particular calculators,, timepieces, and the like. These liquid crystal display elements may be driven by various driving methods. A multiplexing drive is generally used to drive the liquid crystal displays, such as the generalized AC amplitudes selective multiplexing method. However, such a method restricts the maximum number of rows which may be driven between eight and ten as a practical matter. Thus, there is difficulty in using the generalized AC amplitude selective multiplexing method to drive a televisional character display which are operated by addressing the multiplex matrix.
Recently, the two-frequency matrix-addressing method has been found to take advantage of the dielectric dispersion in the liquid crystal material and is somewhat effective in overcoming this disadvantage. However, when the multiplex matrix is addressed, by using the two-frequency matrix-addressing method, the energy consumption is high due to the fact that AC applied voltage is of high frequency and of high voltage. Thus, the two-frequency matrix-addressing method is less than completely satisfactory. It has been found that this energy consumption may be effectively reduced by making the driving voltage lower. It is known that the driving voltage V is dependent upon the dielectric anisotropy .DELTA..epsilon. of the liquid crystal material used. This relationship has been defined as ##EQU1## In other words, as the absolute value ##EQU2## increases, the value of driving voltage V is reduced.
In the liquid crystal materials exhibiting a dielectric dispersion, the dielectric anisotropy .DELTA..epsilon. is positive at relatively low frequencies and is reversed in the high frequency range. This characteristic is exhibited in FIGS. 4-6. The frequency which is 0 is denoted the critical frequency f.sub.c and is a physical characteristic of a liquid crystam material. The particular feature of the two-frequency driving method is that two AC power sources are utilized. A first lower frequency lower than f.sub.c and a second higher frequency, higher than f.sub.c are applied as a driving power source. This takes advantage of the difference in behavior of the liquid crystal material in both the low and high frequency ranges.
Generally speaking, when driving a display device by the multiplex driving method, the ratio of the signal voltage applied in a lighted condition to the signal applied in the non-lighted condition requires a value larger than a certain value, the larger the better. In the case of the generalized AC amplitude selective multiplexing method, the voltage ratio depends only on the number of rows in the device and there is little room for improvement of the display. In contrast, in the two-frequency matrix-addressing method, the ratio depends on the driving voltage and the dielectric constant of the liquid crystal material. It logically follows that any number of rows can be driven by this method.
Liquid crystal materials utilized in liquid crystal display devices driven by these two-frequency matrix-addressing method require the following properties. First, as noted above, in order to reduce the driving voltage, the absolute value of the dielectric anisotropy in both the low and high frequency ranges should be increased. Specifically, the absolute value of the dielectric anisotropy in the high frequency range should be increased. Second, the critical frequency and viscosity of the liquid crystal should be low. Conventional liquid crystal materials do not satisfy these requirements fully.
Accordingly, it is desirable to provide a liquid crystal composition having the desired characteristics. Such a liquid crystal composition would have an increased absolute value of the negative dielectric anisotropy at frequencies higher in the high frequency range.